HList to nested Map

Question

I would like to transform an HList type parameter to a nested Map-type, e.g. Int :: String :: String :: HNil should become Map[Int, Map[String, Map[String, T]]]] where T would be another type parameter of the same function, like:

def somedef[T, L <: HList](t: T)(implicit f: ???): f.Out

where f.Out is T in case of HNil or a nested Map-structure with dept L.size

Is there any way this can be done?

Answer 1

I'm not aware of a standard thing to do such a transformation, but you could roll out your custom converter in the same way as various HList ops (like map) are implemented inside shapeless (see trait Mapper). The code could be something like this:

import scala.language.higherKinds
import scala.collection.immutable.Map
import shapeless._

sealed trait HListToMap[L <: HList, T] {
  type Out

  def convert(hlist: L, value: T): Out
}

object HListToMap {

  // public interface
  def wrap[L <: HList, T](value: T, keys: L)(implicit converter: HListToMap[L, T]): converter.Out =
    converter.convert(keys, value)


  // implementation details
  type Aux[L <: HList, T, Out2] = HListToMap[L, T] { type Out = Out2 }

  private trait Impl[L <: HList, T, Out2] extends HListToMap[L, T] {
    override type Out = Out2
  }

  implicit def hnil[T]: Aux[HNil, T, T] = new Impl[HNil, T, T] {
    override def convert(hlist: HNil, value: T): T = value
  }

  implicit def hnil2[T]: Aux[HNil.type, T, T] = new Impl[HNil.type, T, T] {
    override def convert(hlist: HNil.type, value: T): T = value
  }

  implicit def recurse[H, L <: HList, T](implicit inner: HListToMap[L, T]): Aux[H :: L, T, Map[H, inner.Out]] = new Impl[H :: L, T, Map[H, inner.Out]] {
      override def convert(hlist: H :: L, value: T): Map[H, inner.Out] = {
        val im = inner.convert(hlist.tail, value)
        Map(hlist.head -> im)
      }
    }

}

def test(): Unit = {
  val keys = "abc" :: 1 :: 0.5 :: HNil
  val value = "Xyz"
  val m: Map[String, Map[Int, Map[Double, String]]] = HListToMap.wrap(value, keys)
  println(m)
  val just: String = HListToMap.wrap(value, HNil)
  println(just)
}

You can see it online